Delivery of a shock pulse during the vulnerable period of the cardiac cycle can induce fibrillation, providing the shock energy is greater than a patient-specific minimum value and less than a patient-specific maximum value. Such a shock pulse is generally referred to as a “T-shock” or “T-wave shock” because the time of the vulnerable period during the cardiac cycle generally corresponds to the T-wave of the ECG signal. The upper limit of vulnerability (ULV) for an individual patient is the shock strength at or above which fibrillation is not induced when a shock is delivered during the vulnerable period of a normal cardiac cycle. The minimum shock strength required to defibrillate the human heart, often referred to as the defibrillation threshold (DFT), corresponds quantitatively to the ULV.
In past practice, patients receiving an implantable cardioverter defibrillator (ICD) have undergone DFT testing in order to ensure a reasonable certainty of successful defibrillation using shock pulse energies below the maximum output capacity of the ICD. The DFT has been determined by inducing fibrillation through delivery of a shock during the T-wave, then delivering defibrillation shocks to verify successful defibrillation at shock energies at least a safety margin below the maximum ICD output.
Determination of the ULV as a surrogate for DFT affords the opportunity to significantly reduce the number of times needed to induce fibrillation in a patient in order to ensure the patient's DFT falls below the maximum ICD output. The ULV, which can be measured in regular rhythm, corresponds to a shock strength that defibrillates with a high probability of success.
However, a challenge remains in determining the proper timing of a T-shock delivered to determine the ULV. If a T-shock is properly timed during the vulnerable period, and is greater than or equal to the ULV, fibrillation will not be induced. However, if a T-shock that is below the ULV is delivered just outside the vulnerable period, the resultant failure to induce fibrillation may lead to an incorrect determination of the ULV. Correct timing of T-shocks during the vulnerable period can be determined using 12-lead ECG signals by manually measuring the time interval between a test pacing pulse and a selected point on the T-wave. The ICD is then programmed to deliver a shock at that time interval. However, such techniques using 12-lead ECG signals are time-consuming and require considerable skill. A need remains, therefore, for automated methods for applying shocks at appropriate times relative to the T-wave for reliable ULV determination and DFT estimation.